<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>P0995R0: Improving atomic_flag</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li             { font-weight: bold;   }
	.toc > li li          { font-weight: normal; }
	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @supports not (display:grid) { */
		.toc > li             { margin: 1.5rem 0;    }
		.toc > li li          { margin: 0.3rem 0;    }
		.toc > li li li       { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}

		.toc li {
			clear: both;
		}

		:not(li) > .toc              { margin-left:  5rem; }
		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc              { margin-left:  4rem; }
			.toc .secno                  { margin-left: -4rem; }
			.toc > li li li              { margin-left:  1rem; }
			.toc > li li li .secno       { margin-left: -5rem; }
			.toc > li li li li .secno    { margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good table positioning:
		   "content column" is 50ems wide at max; less on smaller screens.
		   Extra space (after ToC + content) is empty on the right.

		   1. When table < content column, centers table in column.
		   2. When content < table < available, left-aligns.
		   3. When table > available, fills available + scroll bar.
		*/ 
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style type="text/css">
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }

    del { background: #fcc; color: #000; text-decoration: line-through; }
    ins { background: #cfc; color: #000; }
    blockquote .highlight:not(.idl) { background: initial; margin: initial; padding: 0.5em }
    blockquote ul { background: inherit; }
    blockquote code.highlight:not(.idl) { padding: initial; }
    blockquote c-[a] { color: inherit; } /* Keyword.Declaration */
    blockquote c-[b] { color: inherit; } /* Keyword.Type */
    blockquote c-[c] { color: inherit; } /* Comment */
    blockquote c-[d] { color: inherit; } /* Comment.Multiline */
    blockquote c-[e] { color: inherit; } /* Name.Attribute */
    blockquote c-[f] { color: inherit; } /* Name.Tag */
    blockquote c-[g] { color: inherit; } /* Name.Variable */
    blockquote c-[k] { color: inherit; } /* Keyword */
    blockquote c-[l] { color: inherit; } /* Literal */
    blockquote c-[m] { color: inherit; } /* Literal.Number */
    blockquote c-[n] { color: inherit; } /* Name */
    blockquote c-[o] { color: inherit; } /* Operator */
    blockquote c-[p] { color: inherit; } /* Punctuation */
    blockquote c-[s] { color: inherit; } /* Literal.String */
    blockquote c-[t] { color: inherit; } /* Literal.String.Single */
    blockquote c-[u] { color: inherit; } /* Literal.String.Double */
    blockquote c-[cp] { color: inherit; } /* Comment.Preproc */
    blockquote c-[c1] { color: inherit; } /* Comment.Single */
    blockquote c-[cs] { color: inherit; } /* Comment.Special */
    blockquote c-[kc] { color: inherit; } /* Keyword.Constant */
    blockquote c-[kn] { color: inherit; } /* Keyword.Namespace */
    blockquote c-[kp] { color: inherit; } /* Keyword.Pseudo */
    blockquote c-[kr] { color: inherit; } /* Keyword.Reserved */
    blockquote c-[ld] { color: inherit; } /* Literal.Date */
    blockquote c-[nc] { color: inherit; } /* Name.Class */
    blockquote c-[no] { color: inherit; } /* Name.Constant */
    blockquote c-[nd] { color: inherit; } /* Name.Decorator */
    blockquote c-[ni] { color: inherit; } /* Name.Entity */
    blockquote c-[ne] { color: inherit; } /* Name.Exception */
    blockquote c-[nf] { color: inherit; } /* Name.Function */
    blockquote c-[nl] { color: inherit; } /* Name.Label */
    blockquote c-[nn] { color: inherit; } /* Name.Namespace */
    blockquote c-[py] { color: inherit; } /* Name.Property */
    blockquote c-[ow] { color: inherit; } /* Operator.Word */
    blockquote c-[mb] { color: inherit; } /* Literal.Number.Bin */
    blockquote c-[mf] { color: inherit; } /* Literal.Number.Float */
    blockquote c-[mh] { color: inherit; } /* Literal.Number.Hex */
    blockquote c-[mi] { color: inherit; } /* Literal.Number.Integer */
    blockquote c-[mo] { color: inherit; } /* Literal.Number.Oct */
    blockquote c-[sb] { color: inherit; } /* Literal.String.Backtick */
    blockquote c-[sc] { color: inherit; } /* Literal.String.Char */
    blockquote c-[sd] { color: inherit; } /* Literal.String.Doc */
    blockquote c-[se] { color: inherit; } /* Literal.String.Escape */
    blockquote c-[sh] { color: inherit; } /* Literal.String.Heredoc */
    blockquote c-[si] { color: inherit; } /* Literal.String.Interpol */
    blockquote c-[sx] { color: inherit; } /* Literal.String.Other */
    blockquote c-[sr] { color: inherit; } /* Literal.String.Regex */
    blockquote c-[ss] { color: inherit; } /* Literal.String.Symbol */
    blockquote c-[vc] { color: inherit; } /* Name.Variable.Class */
    blockquote c-[vg] { color: inherit; } /* Name.Variable.Global */
    blockquote c-[vi] { color: inherit; } /* Name.Variable.Instance */
    blockquote c-[il] { color: inherit; } /* Literal.Number.Integer.Long */
  </style>
  <link href="http://wg21.link/P0995r0" rel="canonical">
  <link href="https://isocpp.org/favicon.ico" rel="icon">
<style>
ins .highlight:not(.idl) { background: rgba(0, 136, 0, 0.2); }
</style>
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-syntax-highlighting */

.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0995R0<br>Improving atomic_flag</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="1970-01-01">1970-01-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0995r0">http://wg21.link/P0995r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ogiroux@nvidia.com">Olivier Giroux</a> (<span class="p-org org">NVIDIA</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ahh@google.com">Andrew Hunter</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>SG1, LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P0995r0.bs">github.com/jfbastien/papers/blob/master/source/P0995r0.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>atomic_flag is marginally useful. Improve it in light of the new wait / notify APIs.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#word"><span class="secno">2</span> <span class="content">Wording</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>C++11 added <code class="highlight"><c- n>atomic_flag</c-></code> to the language as the minimally-required class which
could be used to implement <code class="highlight"><c- n>atomic</c-><c- o>&lt;></c-></code> on hardware which seemed relevant at the
time. Detailed <code class="highlight"><c- n>atomic_flag</c-></code> history can be found in <a data-link-type="biblio" href="#biblio-n2145">[N2145]</a>, <a data-link-type="biblio" href="#biblio-n2324">[N2324]</a>, and <a data-link-type="biblio" href="#biblio-n2393">[N2393]</a>. The specification was quite successful at minimalism—the only member
functions of <code class="highlight"><c- n>atomic_flag</c-></code> are <code class="highlight"><c- n>test_and_set</c-></code> and <code class="highlight"><c- n>clear</c-></code>—but <code class="highlight"><c- n>atomic</c-><c- o>&lt;></c-></code> was
wildly more successful and to our knowledge has always been implemented with
compiler support instead of with the very inefficient (but beautifully simple) <code class="highlight"><c- n>atomic_flag</c-></code>. Our experience is that <code class="highlight"><c- n>atomic_flag</c-></code>'s interface is so minimal as
to be mostly useless, in particular it doesn’t have a method which can load the
flag’s value without modifying it.</p>
   <p>We’ve heard of it being used as:</p>
   <ul>
    <li data-md>
     <p>A questionable spinloop (as was originally intended);</p>
    <li data-md>
     <p>A "check-in" flag used to know when at least one thread has reached a
program location.</p>
   </ul>
   <p>The one special power <code class="highlight"><c- n>atomic_flag</c-></code> has is in being the only type which is
guaranteed to be lock-free, albeit a mostly powerless one.</p>
   <p>SG1 tried to salvage <code class="highlight"><c- n>atomic_flag</c-></code> in <a data-link-type="biblio" href="#biblio-p0514r0">[P0514R0]</a> by adding <code class="highlight"><c- n>set</c-></code>, <code class="highlight"><c- n>test</c-></code>, <code class="highlight"><c- n>wait</c-></code>, <code class="highlight"><c- n>wait_until</c-></code>, and <code class="highlight"><c- n>wait_for</c-></code> methods but decided to leave it as-is and
implement efficient waiting differently, eventually going for <a data-link-type="biblio" href="#biblio-p0514r3">[P0514R3]</a>.</p>
   <p>The time has come to thank <code class="highlight"><c- n>atomic_flag</c-></code> for serving its purpose as an
implementability stand-in, and help it find its true purpose. We propose:</p>
   <ul>
    <li data-md>
     <p>Adding a <code class="highlight"><c- n>test</c-></code> method to it as <a data-link-type="biblio" href="#biblio-p0514r0">[P0514R0]</a> did. This could technically
forbids some ancestral processors from implementing modern C++, but these
platforms already don’t support any C++.</p>
    <li data-md>
     <p>Add <code class="highlight"><c- n>atomic_flag</c-></code> overloads to <a data-link-type="biblio" href="#biblio-p0514r3">[P0514R3]</a>'s waiting and notify functions.</p>
    <li data-md>
     <p>Add optional always-lock-free integral type aliases, which are encouraged to
be sized such that waiting on them is most efficient.</p>
   </ul>
   <p>This paper was written in Jacksonville and presented to SG1, which unanimously
forwarded the paper to LEWG. LEWG looked at the paper and took the following
poll:</p>
   <table class="def">
    <tbody>
     <tr>
      <th>
      <th><strong>SF</strong>
      <th><strong>F</strong>
      <th><strong>N</strong>
      <th><strong>A</strong>
      <th><strong>SA</strong>
     <tr>
      <th>Make the type aliases non-optional.
      <th>1
      <th>4
      <th>4
      <th>2
      <th>2
   </table>
   <p>The types were made optional in case an architecture, such as PA-RISC, cannot
support always-lock-free integral types because no compare-and-exchange
instruction is available. There was no consensus for making aliases required,
though concern was expressed that LEWG doesn’t usually make functionality
optional. In the C++ standard library optionality is present as follows:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>intN_t</c-></code> / <code class="highlight"><c- n>uintN_t</c-></code> are mandated by C, "if an implementation provides integer
types with widths of 8, 16, 32, or 64 bits, no padding bits, and (for the
signed types) that have a two’s complement representation";</p>
    <li data-md>
     <p><code class="highlight"><c- n>abs</c-></code> and <code class="highlight"><c- n>div</c-></code> overloads "if and only if the type <code class="highlight"><c- b>intmax_t</c-></code> designates an
extended integer type";</p>
    <li data-md>
     <p>The library <code class="highlight"><c- n>allocator_traits</c-></code> template has optional requirements.</p>
   </ul>
   <p>There was also discussion about ABI breakage to <code class="highlight"><c- n>atomic_flag</c-></code>. An argument was
made that <code class="highlight"><c- n>atomic_flag</c-></code> should also be sized such that waiting on them is most
efficient (which would be an ABI breakage), and if that breakage doesn’t occur
then adding wait / notify overloads is actively misleading. LEWG want SG1 to
reconsider whether the overloads should be provided.</p>
   <h2 class="heading settled" data-level="2" id="word"><span class="secno">2. </span><span class="content">Wording</span><a class="self-link" href="#word"></a></h2>
   <p>Under Header <code class="highlight"><c- o>&lt;</c-><c- n>atomic</c-><c- o>></c-></code> synopsis [<strong>atomics.syn</strong>] edit as follows:</p>
   <blockquote>
<pre class="highlight"><c- c1>// 32.3, type aliases</c->

<c- c1>// ...</c->
</pre>
    <ins>
<pre class="highlight"><c- k>using</c-> <c- n>atomic_signed_lock_free</c->   <c- o>=</c-> <c- n>see</c-> <c- n>below</c-><c- p>;</c-> <c- c1>// optional</c->
<c- k>using</c-> <c- n>atomic_unsigned_lock_free</c-> <c- o>=</c-> <c- n>see</c-> <c- n>below</c-><c- p>;</c-> <c- c1>// optional</c->
</pre>
    </ins>
<pre class="highlight"><c- c1>// 32.8, flag type and operations</c->
<c- k>struct</c-> <c- n>atomic_flag</c-><c- p>;</c->
</pre>
    <ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c-> <c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- cp>#define ATOMIC_FLAG_INIT see below</c->
</pre>
<pre class="highlight"><c- c1>// 32.10, waiting and notifying functions</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>);</c->
</pre>
    </ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>);</c->
</pre>
    </ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>,</c->
                   <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>,</c-> <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- b>bool</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- b>bool</c-><c- p>);</c->
</pre>
    </ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>,</c->
                            <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-><c- p>,</c->
                            <c- n>memory_order</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-><c- p>,</c->
                            <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- b>bool</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- b>bool</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>);</c->
</pre>
    </ins>
   </blockquote>
   <p>In Atomic operations library [<strong>atomics</strong>], under Type aliases
[<strong>atomics.alias</strong>], edit as follows:</p>
   <blockquote>
    <p>The type aliases <code class="highlight"><c- n>atomic_intN_t</c-></code>, <code class="highlight"><c- n>atomic_uintN_t</c-></code>, <code class="highlight"><c- n>atomic_intptr_t</c-></code>, and <code class="highlight"><c- n>atomic_uintptr_t</c-></code> are defined if and only if <code class="highlight"><c- n>intN_t</c-></code>, <code class="highlight"><c- n>uintN_t</c-></code>, <code class="highlight"><c- b>intptr_t</c-></code>,
and <code class="highlight"><c- b>uintptr_t</c-></code> are defined, respectively.</p>
    <ins>
     <p>The type aliases <code class="highlight"><c- n>atomic_signed_lock_free</c-></code> and <code class="highlight"><c- n>atomic_unsigned_lock_free</c-></code> are
defined to be specializations of <code class="highlight"><c- n>atomic</c-></code> whose template arguments are integral
types, respectively signed and unsigned, other than <code class="highlight"><c- b>bool</c-></code>. <code class="highlight"><c- n>is_always_lock_free</c-></code> shall be <code class="highlight">true</code> for <code class="highlight"><c- n>atomic_signed_lock_free</c-></code> and <code class="highlight"><c- n>atomic_unsigned_lock_free</c-></code>. These aliases are optional. If an implementation
provides a integral specialization of <code class="highlight"><c- n>atomic</c-></code> other than <code class="highlight"><c- b>bool</c-></code> for which <code class="highlight"><c- n>is_always_lock_free</c-></code> is <code class="highlight">true</code>, it shall define the aliases to such a
type. Otherwise, they shall not be defined. An implementation should choose the
integral specialization of <code class="highlight"><c- n>atomic</c-></code> for which the waiting and notifying
functions are most efficient.</p>
    </ins>
   </blockquote>
   <p>In Atomic operations library [<strong>atomics</strong>], under Flag type and operations
[<strong>atomics.flag</strong>], edit as follows:</p>
   <blockquote>
<pre class="highlight"><c- k>namespace</c-> <c- n>std</c-> <c- p>{</c->
  <c- k>struct</c-> <c- n>atomic_flag</c-> <c- p>{</c->
</pre>
    <ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>test</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>test</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
<pre class="highlight">  <c- b>bool</c-> <c- nf>test_and_set</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>bool</c-> <c- nf>test_and_set</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>clear</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>clear</c-><c- p>(</c-><c- n>memory_order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- n>atomic_flag</c-><c- p>()</c-> <c- k>noexcept</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
  <c- n>atomic_flag</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
  <c- n>atomic_flag</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>&amp;</c-><c- p>)</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
  <c- n>atomic_flag</c-><c- o>&amp;</c-> <c- k>operator</c-><c- o>=</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>&amp;</c-><c- p>)</c-> <c- k>volatile</c-> <c- o>=</c-> <c- k>delete</c-><c- p>;</c->
<c- p>};</c->
</pre>
    <ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    </ins>
<pre class="highlight">  <c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-><c- p>,</c-> <c- n>memory_order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
  <c- cp>#define ATOMIC_FLAG_INIT see below</c->
<c- p>}</c->
</pre>
    <p>The <code class="highlight"><c- n>atomic_flag</c-></code> type provides the classic test-and-set functionality. It has
two states, set and clear.</p>
    <p>Operations on an object of type <code class="highlight"><c- n>atomic_flag</c-></code> shall be lock-free. [ <em>Note:</em> Hence the operations should also be address-free. <em>—end note</em>]</p>
    <p>The <code class="highlight"><c- n>atomic_flag</c-></code> type is a standard-layout struct. It has a trivial default
constructor and a trivial destructor.</p>
    <p>The macro <code class="highlight"><c- n>ATOMIC_FLAG_INIT</c-></code> shall be defined in such a way that it can be used to initialize an object of
type <code class="highlight"><c- n>atomic_flag</c-></code> to the clear state. The macro can be used in the form:</p>
<pre class="highlight"><c- n>atomic_flag</c-> <c- n>guard</c-> <c- o>=</c-> <c- n>ATOMIC_FLAG_INIT</c-><c- p>;</c-></pre>
    <p>It is unspecified whether the macro can be used in other initialization
contexts. For a complete static-duration object, that initialization shall be
static. Unless initialized with <code class="highlight"><c- n>ATOMIC_FLAG_INIT</c-></code>, it is unspecified whether an <code class="highlight"><c- n>atomic_flag</c-></code> object has an initial state of set or clear.</p>
    <ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>test</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>test</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
     <p><em>Requires:</em> The <code class="highlight"><c- n>order</c-></code> argument shall not be <code class="highlight"><c- n>memory_order_release</c-></code> nor <code class="highlight"><c- n>memory_order_acq_rel</c-></code>.</p>
     <p><em>Effects:</em> Memory is affected according to the value of <code class="highlight"><c- n>order</c-></code>.</p>
     <p><em>Returns:</em> Atomically returns the value pointed to by <code class="highlight"><c- n>object</c-></code> or <code class="highlight"><c- k>this</c-></code>.</p>
    </ins>
<pre class="highlight"><c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_and_set</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- nf>atomic_flag_test_and_set_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>test_and_set</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>bool</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>test_and_set</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p><em>Effects:</em> Atomically sets the value pointed to by <code class="highlight"><c- n>object</c-></code> or by <code class="highlight"><c- k>this</c-></code> to <code class="highlight">true</code>. Memory is affected according to the value of <code class="highlight"><c- n>order</c-></code>. These operations
are atomic read-modify-write operations (4.7).</p>
    <p><em>Returns:</em> Atomically, the value of the object immediately before the effects.</p>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- nf>atomic_flag_clear_explicit</c-><c- p>(</c-><c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>clear</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>volatile</c-> <c- k>noexcept</c-><c- p>;</c->
<c- b>void</c-> <c- n>atomic_flag</c-><c- o>::</c-><c- n>clear</c-><c- p>(</c-><c- n>memory_order</c-> <c- n>order</c-> <c- o>=</c-> <c- n>memory_order_seq_cst</c-><c- p>)</c-> <c- k>noexcept</c-><c- p>;</c->
</pre>
    <p><em>Requires:</em> The <code class="highlight"><c- n>order</c-></code> argument shall not be <code class="highlight"><c- n>memory_order_consume</c-></code>, <code class="highlight"><c- n>memory_order_acquire</c-></code>, nor <code class="highlight"><c- n>memory_order_acq_rel</c-></code>.</p>
    <p><em>Effects:</em> Atomically sets the value pointed to by <code class="highlight"><c- n>object</c-></code> or by <code class="highlight"><c- k>this</c-></code> to <code class="highlight">false</code>. Memory is affected according to the value of <code class="highlight"><c- n>order</c-></code>.</p>
   </blockquote>
   <p>In Atomic operations library [<strong>atomics</strong>], under Waiting and notifying
functions [<strong>atomics.wait</strong>], edit as follows:</p>
   <blockquote>
    <p>The functions in this subclause provide a mechanism to wait for the value of an
atomic object to change, more efficiently than can be achieved with polling.
Waiting functions in this facility may block until they are unblocked by
notifying functions, according to each function’s effects. [<em>Note:</em> Programs
are not guaranteed to observe transient atomic values, an issue known as the
A-B-A problem, resulting in continued blocking if a condition is only
temporarily met. <em>– End Note.</em>]</p>
    <p>The functions <code class="highlight"><c- n>atomic_wait</c-></code> and <code class="highlight"><c- n>atomic_wait_explicit</c-></code> are waiting
functions. The functions <code class="highlight"><c- n>atomic_notify_one</c-></code> and <code class="highlight"><c- n>atomic_notify_all</c-></code> are
notifying functions.</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_notify_one</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>);</c->
</pre>
    </ins>
    <p><em>Effects:</em> unblocks up to execution of a waiting function that blocked after
observing the result of an atomic operation X, if there exists another atomic
operation Y, such that X precedes Y in the modification order of <code class="highlight"><c- o>*</c-><c- n>object</c-></code>, and
Y happens-before this call.</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_notify_all</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>);</c->
</pre>
    </ins>
    <p><em>Effects:</em> unblocks each execution of a waiting function that blocked after
observing the result of an atomic operation X, if there exists another atomic
operation Y, such that X precedes Y in the modification order of <code class="highlight"><c- o>*</c-><c- n>object</c-></code>, and
Y happens-before this call.</p>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>,</c->
                            <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-> <c- n>old</c-><c- p>,</c->
                            <c- n>memory_order</c-> <c- n>order</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>,</c->
                            <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-> <c- n>old</c-><c- p>,</c->
                            <c- n>memory_order</c-> <c- n>order</c-><c- p>);</c->
</pre>
    <p><em>Requires:</em> The order argument shall not be <code class="highlight"><c- n>memory_order_release</c-></code> nor <code class="highlight"><c- n>memory_order_acq_rel</c-></code>.</p>
    <p><em>Effects:</em> Repeatedly performs the following steps, in order:</p>
    <ol>
     <li data-md>
      <p>Evaluates <code class="highlight"><c- n>object</c-><c- o>-></c-><c- n>load</c-><c- p>(</c-><c- n>order</c-><c- p>)</c-> <c- o>!=</c-> <c- n>old</c-></code> then, if the result is <code class="highlight">true</code>, returns.</p>
     <li data-md>
      <p>Blocks until an implementation-defined condition has been met. [<em>Note:</em> Consequently, it may unblock for reasons other than a call to a notifying
 function. <em>- end note</em>]</p>
    </ol>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- b>bool</c-> <c- n>old</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_wait_explicit</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- b>bool</c-> <c- n>old</c-><c- p>,</c-> <c- n>memory_order</c-> <c- n>order</c-><c- p>);</c->
</pre>
     <p><em>Effects:</em> Repeatedly performs the following steps, in order:</p>
     <ol>
      <li data-md>
       <p>Evaluates <code class="highlight"><c- n>object</c-><c- o>-></c-><c- n>test</c-><c- p>(</c-><c- n>order</c-><c- p>)</c-> <c- o>!=</c-> <c- n>old</c-></code> then, if the result is <code class="highlight">true</code>, returns.</p>
      <li data-md>
       <p>Blocks until an implementation-defined condition has been met. [<em>Note:</em> Consequently, it may unblock for reasons other than a call to a notifying
 function. <em>- end note</em>]</p>
     </ol>
    </ins>
<pre class="highlight"><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>,</c->
                   <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-> <c- n>old</c-><c- p>);</c->
<c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c->
  <c- b>void</c-> <c- n>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>*</c-> <c- n>object</c-><c- p>,</c->
                   <c- k>typename</c-> <c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value_type</c-> <c- n>old</c-><c- p>);</c->
</pre>
    <ins>
<pre class="highlight"><c- b>void</c-> <c- nf>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- k>volatile</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- b>bool</c-> <c- n>old</c-><c- p>);</c->
<c- b>void</c-> <c- nf>atomic_wait</c-><c- p>(</c-><c- k>const</c-> <c- n>atomic_flag</c-><c- o>*</c-> <c- n>object</c-><c- p>,</c-> <c- b>bool</c-> <c- n>old</c-><c- p>);</c->
</pre>
    </ins>
    <p><em>Effects:</em> Equivalent to: <code class="highlight"><c- n>atomic_wait_explicit</c-><c- p>(</c-><c- n>object</c-><c- p>,</c-> <c- n>old</c-><c- p>,</c-> <c- n>memory_order_seq_cst</c-><c- p>);</c-></code></p>
   </blockquote>
   <p>Two feature test macros should be added:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>__cpp_lib_atomic_flag_test</c-></code> implies the <code class="highlight"><c- n>test</c-></code> methods for <code class="highlight"><c- n>atomic_flag</c-></code> and free functions, as well as the notify and wait overloads for <code class="highlight"><c- n>atomic_flag</c-></code>, are available.</p>
    <li data-md>
     <p><code class="highlight"><c- n>__cpp_lib_atomic_lock_free_type_aliases</c-></code> implies <code class="highlight"><c- n>atomic_signed_lock_free</c-></code> and <code class="highlight"><c- n>atomic_unsigned_lock_free</c-></code> types are defined.</p>
   </ul>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don’t scroll to compensate for the ToC if we’re above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don’t know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn’t exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn’t standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don’t have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we’re at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-n2145">[N2145]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2145">C++ Atomic Types and Operations</a>. 12 January 2007. URL: <a href="https://wg21.link/n2145">https://wg21.link/n2145</a>
   <dt id="biblio-n2324">[N2324]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2324">C++ Atomic Types and Operations</a>. 24 June 2007. URL: <a href="https://wg21.link/n2324">https://wg21.link/n2324</a>
   <dt id="biblio-n2393">[N2393]
   <dd>H.-J. Boehm, L. Crowl. <a href="https://wg21.link/n2393">C++ Atomic Types and Operations</a>. 10 September 2007. URL: <a href="https://wg21.link/n2393">https://wg21.link/n2393</a>
   <dt id="biblio-p0514r0">[P0514R0]
   <dd>Olivier Giroux. <a href="https://wg21.link/p0514r0">Enhancing std::atomic_flag for waiting</a>. 15 November 2016. URL: <a href="https://wg21.link/p0514r0">https://wg21.link/p0514r0</a>
   <dt id="biblio-p0514r3">[P0514R3]
   <dd>Olivier Giroux. <a href="https://wg21.link/p0514r3">Efficient concurrent waiting for C++20</a>. 10 February 2018. URL: <a href="https://wg21.link/p0514r3">https://wg21.link/p0514r3</a>
  </dl>